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9 Oct 2014 Agenda Objectives: Atomic number Mass number Isotopes Average Atomic Mass Slip Quiz 1 Isotopes – Are all atoms of an element alike? Review Extension and Electrons in Atoms Questions Average Atomic Mass – POGIL/notes – calculators/rulers Homework Slip Quiz Part 1 1. Define mass number of an atom and state why it is called the “mass number.” 2. Which of the following pairs show 2 atoms with the same number of neutrons? (Look up and write in the appropriate atomic numbers.) 37Cl and 38Ar 59Co and 61Co 32P and 32S 65Zn and 67Zn Slip Quiz Part 1 1. Define mass number of an atom and state why it is called the “mass number.” Mass number is defined as the sum of the number of protons and neutrons in the nucleus of the atom. It is called mass number because the protons and neutrons carry most of the mass of an atom, and so the total number of them gives an idea of what the mass of the atom itself will be. Slip Quiz Part 1 Which of the following pairs show 2 atoms with the same number of neutrons? (Look up and write in the appropriate atomic numbers.) 37Cl and 38Ar 59Co and 61Co 2. 17 32P 15 18 and 32S 16 27 65Zn 30 27 and 67Zn 30 Slip Quiz Part 1 Which of the following pairs show 2 atoms with the same number of neutrons? (Look up and write in the appropriate atomic numbers.) 37Cl and 38Ar 59Co and 61Co 2. 17 32P 15 18 and 32S 16 27 65Zn 30 27 and 67Zn 30 Isotopes- Last Page Electrons in Atoms and Extension Questions 14. The number of electrons in each of the atomic diagrams in Model 1 – is the same as the number of protons in the nucleus of the atomic diagrams (even if they are difficult to spot.) a. In a neutral atom, how does the number of electrons compare to the number of protons. The number of electrons is equal to the number of protons. Isotopes Electrons in Atoms and Extension Questions 14b. Atoms are “neutral” meaning they have no overall electrostatic charge. The positive charge in the nucleus caused by the presence of protons is “cancelled” out (when we view the atom as a whole) by the same number of negatively charged electrons in the electron cloud around that nucleus. xp+ + xe- = 0 Isotopes Electrons in Atoms andExtension Questions 16. Can two atoms with the same mass number ever be isotopes of each other? Explain. If two atoms have the same mass number, they could be identical atoms with the same atomic number and mass number. In this case they would not be isotopes of each other. If two atoms have different atomic numbers and just happen to have the same mass numbers, then again they would not be isotopes of each other. Isotopes Electrons in Atoms and Extension Questions 17. All models have limitations. What characteristics of Model 1 are inconsistent with your understanding of what atoms look like? • Electrons are much smaller than protons and neutrons yet in Model 1 they are all represented by similar sized symbols. • Electrons are much further from the nucleus than represented in Model 1. • Atoms and all the subatomic particles are 3Dimensional and not flat. Average Atomic Mass How are the masses on the periodic table determined? Why? Most elements have more than one naturally occurring isotope. As you learned previously, the atoms of those isotopes have the same atomic number (number of protons), making them belong to the same element, but they have different mass numbers (total number of protons and neutrons) giving them different atomic masses. Average Atomic Mass How are the masses on the periodic table determined? So which mass is put on the periodic table for each element? Is it the most common isotope’s mass? The heaviest mass? This activity will help you answer that question. Qu 1 – 6 together. 7a. Expected number of atoms of each isotope in a sample of 20 atoms (whole numbers for atoms) Mg-24: 78.99 x 20 = 100 Mg-24: 78.99 x 20 = 15.8 = 16 100 Mg-25: 10.00 x 20 = 2.0 = 2 100 Mg-26: 11.01 x 20 = 2.2 = 2 100 Is model 1 accurate in its representation of magnesium at the atomic level? Explain. 8. If you could pick up an atom and put it on a balance, the mass of that atom would most likely be 23.9850 amu because there are more Mg-24 in the sample than any other isotope. The chances are greatest that I would pick up a Mg-24 atom. 9. a. From periodic table mass of Mg is 24.30amu (24.3050amu) b. Does the decimal number shown on the periodic table for magnesium match any of the atomic masses listed in Model 2? No, none of the atomic masses listed in Model 2 match the decimal mass value reported on our periodic tables. 10. The periodic table does not show the atomic mass of every isotope for an element. a) This would be an impractical goal for the periodic table because it would take up too much space to include information about 2, 3 or more isotopes for each element. (It would be confusing to read also.) 10. The periodic table does not show the atomic mass of every isotope for an element. b) It isn’t important to the average scientist to have information about a particular isotope of an element, because in most cases a scientist will be working with a mixture of isotopes and their chemical behavior will be the same. Information about individual isotopes will not be necessary . 11. A practical way of showing the mass of magnesium atoms on the periodic table, given that most elements occur as a mixture of isotopes, would be to calculate an average and report that. 12. A way to calculate the average atomic mass of 100 magnesium atoms. 12. A way to calculate the average atomic mass of 100 magnesium atoms. Add up all the masses of all the atoms in a sample and divide by the number of atoms in the sample, 100. Write in space on back sheet. Atomic Mass Units (amu) The masses of protons, neutrons and electrons in g are very small and difficult to work with. Chemists developed a method of measuring the mass of an atom relative to the mass of a specifically chosen atomic standard. Carbon-12 atom One carbon-12 atom was assigned a mass of exactly 12 atomic mass units (amu). Atomic Mass Units (amu) …atomic standard Carbon-12 atom One carbon-12 atom was assigned a mass of exactly 12 atomic mass units (amu). One atomic mass unit (amu) is defined as 1/12th (one twelfth) of the mass of a carbon12 atom. 1 amu is close to the mass of a single proton or single neutron. See p 102 Table 4-2 Model 3 – Proposed Average Atomic Mass Calculations Mary’s method gives same answer as PT (78.99)(23.9850 amu) + (10.00)(24.9858 amu) + (11.01)(25.9826 amu) 100 = 24.305 amu Jack’s Method – gives same answer as PT (0.7899)(23.9850 amu) + (0.1000)(24.9858 amu) + (0.1101)(25.9826amu) = 24.305 amu Alan’s method – assumes equal representation of all isotopes, only finds average for 3 atoms, one of each isotope (23.9850 amu) + (24.9858 amu) + (25.9826 amu) = 24.984 amu 3 14.c. show Mary and Jack’s methods are mathematically equivalent models. Mary’s method (78.99)(23.9850 amu) + (10.00)(24.9858 amu) + (11.01)(25.9826 amu) 100 =78.99)(23.9850 amu) + (10.00)(24.9858 amu) + (11.01)(25.9826 amu) 100 100 100 =(0.7899)(23.9850 amu) + (0.1000)(24.9858 amu) + (0.1101)(25.9826amu =(0.7899)(23.9850 amu) + (0.1000)(24.9858 amu)+ (0.1101)(25.9826amu) = 24.305 amu Jack’s Method I will look at your answer to questions 15. when you turn in the packet at the end of class or at start of next class meeting. Read This! Recall that all isotopes of an element have the same physical and chemical properties, with the exception of atomic mass (and for unstable isotopes, radioactivity). Therefore, the whole periodic table lists a weighted average atomic mass for each element. In order to calculate this quantity, the natural abundance and atomic mass of each isotope must be provided. 16. Consider the individual atomic masses for magnesium isotopes give in Model 2. a) Which isotope has an atomic mass closest to the average atomic mass listed on the periodic table? 16. Consider the individual atomic masses for magnesium isotopes give in Model 2. a) Which isotope has an atomic mass closest to the average atomic mass listed on the periodic table? 24Mg b) Give a mathematical reason for answer to part a. Weighted average atomic mass cont. 24Mg is the most common isotope and is thus most heavily “weighted” in the equation for average atomic mass. There are more 24Mg atoms in a sample than atoms of the other isotopes since it is the most naturally abundant isotope. The mass of 24Mg atoms contribute the most to the average mass of the sample. Weighted average atomic mass cont. 17. Boron has two naturally occurring isotopes: boron-10 and boron-11. Which isotope is more abundant on Earth? Use grammatically correct sentences to explain the answer. (3 mins to discuss) – share. Weighted average atomic mass cont. The expected mass of boron-10 would be 10 amu and the expected mass of boron -11 would be 11 amu. On the periodic table the average atomic mass for boron is listed as 10.811amu, which is numerically closer to 11 than it is to 10. Therefore we can conclude that boron-11 is more abundant on Earth than boron-10. Homework Turn in assignment using our Google Classroom by 11:59pm Tues Oct. 14th. • Find out the names that 110, 111, 112, 114, and 116 have now been given. What is the latest news about element 118? • Who/what makes the decisions about element names? • How long does it take for a name to be decided upon? • Record your source(s) using MLA formatting. Read your notes about Chromatography for lab Mondayshoes – markers to investigate.